Not Applicable.
Not Applicable.
Not Applicable
1. Field of the Invention
This invention relates generally to automatic test equipment for electronics (xe2x80x9cATExe2x80x9d), and, more particularly, to processes for deskewing digitizer channels that are used to capture waveforms in ATE systems.
2. Description of Related Art
Manufacturers of semiconductor devices commonly use ATE, or xe2x80x9ctesters,xe2x80x9d to test whether devices meet their requirements. Testing devices early in the manufacturing process allows devices that fail their tests to be discarded before additional manufacturing costs are incurred. In addition, sorting devices into different categories based on test results allows manufacturers to sell devices having different performance at different prices.
Testers generally come equipped with instruments called xe2x80x9cdigitizersxe2x80x9d for capturing signals from devices under test (DUTs). Digitizers include hardware for sampling analog signals and converting them to digital records. These records can then be analyzed and tested using a test system computer. Many testers include built-in digitizers, which communicate with the test system computer via an internal (and generally proprietary) instrument bus. Testers can also employ external digitizers, which communicate via an industry standard bus, such as IEEE-488 or VXI.
When different digitizers are used, or, equivalently, when different channels of one or more digitizers are used, a need arises to align these digitizer channels in time. Consider a case in which a first digitizer channel samples the input to a DUT and a second digitizer channel samples the output. If one wishes to measure the DUT""s propagation delay, it is first necessary to know the difference in delay (i.e., xe2x80x9ctiming skewxe2x80x9d) inherent between the first and second digitizer channels and their associated cabling. Only when this difference is known can an accurate value of propagation delay be reported. Timing skew originates from many sources, which include differences in electrical paths leading up to the digitizer channels, differences in characteristic impedance of the paths, and differences in electrical propagation delays within the digitizer channels themselves.
FIG. 1 shows a prior technique for determining timing skew between two digitizer channels. At step 110, a waveform conveying an edge or other electrical event is applied to the inputs of the digitizer channels simultaneously (e.g., using matched length cables and a power splitter). The digitizer channels are each made to capture the waveform (step 112). The tester then examines the data records associated with the digitizer channels to determine the locations of the edge within the records (step 114). The difference between these locations (determined at step 116) corresponds to the timing skew. This value can then be subtracted from any time measurement taken between these digitizer channels to yield a measurement that is corrected for timing skew.
This prior technique is extremely effective; however, it requires averaging before its best accuracy can be attained. Jitter, i.e., timing noise, in the digitizers"" circuitry and in the paths leading to the digitizer channels, gives rise to errors in the measurement of timing skew. Repeating the skew measurement and averaging the results (steps 118 and 120) can substantially reduce these errors. These steps require additional time, however. This prior technique only considers samples in the vicinity of the edge in computing the timing skew. Therefore, many repetitions of the edge are required to substantially reduce the jitter. In addition, each edge must be found by searching the digitizer""s data record for the expected level transition. The time needed to perform these functions negatively impacts tester performance.
Another prior technique for determining timing skew has been to use a TDR (Time Domain Reflectometer) to measure delays in the paths leading up to the digitizer channels. According to this technique, an instrument sends a pulse down a line leading to a digitizer channel and measures the time it takes for the pulse to reflect from the end of the line and return to the source. The delay of the line is then computed as one-half the round-trip transit time of the pulse. This measurement is repeated for each digitizer channel, and skew between the channels is computed.
The TDR technique can accurately measure path delays, but it cannot measure internal delays, such as propagation delays within the digitizer circuitry itself. Circuit delays can represent a significant portion of overall skew, and failing to remove them can yield unsatisfactory results.
What is needed is a deskewing technique that is fast, accurate, and accounts for all the significant sources of skew.
With the foregoing background in mind, it is an object of the invention accurately and quickly to eliminate timing skew between digitizer channels of an automatic test system.
To achieve the foregoing object, as well as other objectives and advantages, a waveform is applied to the input of each of a plurality of digitizer channels. Each digitizer channel samples the waveform to produce a respective data record. A Discrete Fourier Transform (DFT), or a portion thereof, is then taken for each data record to determine, at minimum, the phase of the waveform. Phase differences between different digitizer channels are converted to time differences, which are applied to subsequent measurements to correct for the effects of timing skew.